1. Field of the Invention
The present invention relates to a toner for developing electrostatic image (hereinafter, “a toner for developing electrostatic image” may be simply referred to as “a toner”) used for development of electrostatic image or the like in an electrophotography, an electrostatic recording method, an electrostatic printing process or the like.
2. Description of the Related Art
A method of forming desired images by developing electrostatic image with a toner is widely used.
For example, in an electrophotography, a latent image of electrostatics formed on a photosensitive member is developed by a toner comprising a colored particle and an external additive so as to obtain a toner image, and the toner image is transferred to a transferring material such as paper, an OHP sheet or the like. Then, the toner image is fixed on the transferring material, and thus obtained a printed product.
The need of colorization for image-forming devices employing the electrophotography such as copying machines, facsimiles, printers or the like is increasing. In a full-color electrophotography, color image forming generally uses 3 color toners such as yellow, magenta and cyan or 4 color toners such as yellow, magenta, cyan and black to reproduce colors. An example of the image forming method in the case of color copying is as follows. Firstly, a color image is scanned by being broken into pixels and digital image signals by color are sent to a light radiation device. Light is radiated on a surface of a charged photosensitive member from the light radiation device to form a latent image of electrostatics. Next, the latent image of electrostatics is developed by color toners which correspond to the latent image of electrostatics by color on the photosensitive member and finally transferred to a transferring material such as paper, an OHP sheet or the like.
On the other hand, the toner remaining on the photosensitive member after the transferring process (hereinafter, it may be referred to as “a remaining toner”) is removed through the cleaning process by means of a cleaning device such as a cleaning blade or the like.
Generally, toners used for development are broadly classified into a toner produced by a pulverizing method or a toner produced by a polymerization method.
The pulverizing method is a method of producing a colored particle by pulverizing and classifying a solid of a colored resin obtained by a method of dissolving, mixing and kneading a binder and a colorant or a method of polymerizing a mixture containing a monomer and a colorant.
On the other hand, as the polymerization method, for example, there may be a suspension polymerization method, wherein a droplet of a polymerizable monomer composition containing a polymerizable monomer and a colorant is formed and the droplet is polymerized so as to produce a colored particle, or the like. While the colored particle obtained by the pulverizing method is amorphous, the colored particle obtained by the polymerization method is close to a spherical shape and has a small particle diameter and a sharp particle size distribution.
Particularly, from the viewpoint of improving an image quality such as image reproducibility, fineness or the like, a toner whose shape and particle diameter distribution are highly controlled such as the toner obtained by the polymerization method (a so-called polymerized toner) has been used.
However, when using a toner having a small particle diameter or a toner having a spherical shape, a remaining toner is likely to pass between the photosensitive member and the cleaning blade in the cleaning process. Hence, a cleaning problem that the remaining toner may remain on the photosensitive member after the cleaning process is likely to occur, thereby, filming of the remaining toner on the photosensitive member may occur by continuous image forming and the remaining toner may cause insufficient charging on the surface of the photosensitive member, poor latent image forming, decrease in charge amount of the toner, fogging and so on.
Therefore, development of a toner which is excellent in high image reproducibility and further in cleaning performance is desired.
Japanese Patent Application Laid-Open (JP-A) No. 2005-010723 aims to provide a toner which can be easily cleaned without decreasing reproducibility of thin lines due to toner scattering, and discloses a method of producing a toner for developing electrostatic image, wherein an organic solvent is removed while applying shear stress to an emulsified dispersion liquid of a toner composition. Meanwhile, JP-A No. 2005-010723 discloses that the toner manufactured by the above method has a spindle form and 3 to 8 μm volume average particle diameter.
However, the toner disclosed in JP-A No. 2005-010723 may not be sufficient for reproducibility of thin lines and cleaning performance.
On the other hand, toner flowability is being considered in order to improve characteristics of a toner other than reproducibility of thin lines and cleaning performance mentioned above.
JP-A No. 2003-295498 discloses a toner comprising a coloring particle and an external addictive, wherein an angle of repose of the toner is 15 to 30° and a looseness apparent specific gravity thereof is 0.40 to 0.45 g/cc. Also, JP-A No. 2003-295498 mentions a toner containing a silica microparticle having a volume average particle diameter of a primary particle of 5 to 18 μm and a microparticle having a volume average particle diameter of a primary particle of 0.1 to 1.0 μm as an external additive.
JP-A No. 2004-177747 discloses a toner for developing a latent image of electrostatics comprising an external additive including a silica coated metal oxide particle having a core-shell structure and a silica microparticle having a volume average particle diameter of 5 to 20 nm, and a colored particle.
JP-A No. 2003-330218 discloses in claims a toner having a uniaxial collapsing stress of 50 G [N/m2] (equivalent to 0.5 kPa/g; G means gravity acceleration) or less in order to prevent clogging of the toner in an image forming device transferring the toner by means of an air pump. However, a composition and a production method of the toner are not disclosed in JP-A No. 2003-330218.
The toners mentioned in the above priority arts do not basically consider reproducibility of thin lines and cleaning performance.
The present invention has been achieved in light of the above-stated circumstances. An object of the present invention is to provide a toner which is excellent in image reproductivity and further in cleaning performance.